This invention relates generally to gradient field coil systems for image-generating apparatus which is used in nuclear magnetic resonance tomography, and more particularly, to an arrangement where a gradient coil system is arranged on at least one rotation-symmetrical support body having an axis which is arranged in the z-direction and which has a field gradient which is essentially constant in the imaging region.
In image-generating apparatus for nuclear magnetic resonance technology, particularly for zeugmatography, a gradient field coil system is arranged on a rotation-symmetrical support body for producing in the imaging area a largely constant field gradient G.sub.z =.delta.B.sub.z /.delta.z. This substantially constant field gradient is produced by a respective pair of toroidal individual coils which are at least approximately symmetrical to the x-y plane through the center of the imaging area and which extends in a direction perpendicular to the z-axis. Current flows in opposite directions through the toroidal coils of the pair. A further set of individual coils is arranged substantially symmetrically with respect to the plane of symmetry for generating field gradient G.sub.x =.delta.B.sub.z /.delta.x in the x-direction, which field gradient is essentially constant in the imaging area, and at least a still further set of individual coils for generating a corresponding field gradient G.sub.y =.delta.B.sub.z /.delta.y in the y-direction. Such a gradient coil system is described in DE-OS No. 28 40 178.
In the field of medical diagnostics, imaging methods have been proposed wherein an image similar to an x-ray tomogram is constructed by numerical or measurement analysis of integrated proton resonance signals from the spatial spin density and/or relaxation time distribution of a human body to be examined. The corresponding method is also known as "zeugmatography" or nuclear spin tomography. See: "Nature", volume 242 1973, pages 190 to 191.
According to the known methods of nuclear spin tomography, three different kinds of coil systems are required, in principle. One magnet is required to generate a stationary base field B.sub.z which must be as homogeneous as possible and have an order of magnitude of between 0.05 to 0.5 Tesla. Magnetic field B.sub.z is assumed to be oriented, for example, in the z-direction of an orthogonal x, y, z coordinate system. Moreover, the z-direction is the examination axis along which a body, particularly a human body to be examined, is placed in the magnetic field. The coordinate origin is to be situated in the imaging, or examination region. Furthermore, a high-frequency coil arrangement is to be provided for the corresponding precession frequency of the nuclear spin to be considered, in order to excite the nuclear spin, and optionally, to receive the induction signals. If the high-frequency coil arrangement is used for detecting these signals, a separate receiving coil system may also be provided. Finally, a system of gradient coils is needed which generate a preferably orthogonal set of supplementary fields G.sub.z =.delta.B.sub.z /.delta.z; G.sub.x =.delta. B.sub.z /.delta.x; and G.sub.y 32 .delta.B.sub.z /.delta.y. These supplementary fields are small in comparison with the base field B.sub.z which is oriented in the z-direction. Only the gradient fields which are switched on in the predetermined sequence permit a distinction in the location due to the shape of the precession frequency of the nuclei. See, for example, "Journal of Magnetic Resonance", volume 18, 1975, pages 69 to 83; volume 29, 1978, pages 355 to 373.
If the gradients G.sub.x, G.sub.y, and G.sub.z in an imaging region are not constant to a high degree, but are still functions of the location itself, blurred, distroted, and artificial images are generated. Linearity of the gradient fields and the constancy of their derivatives G.sub.x, G.sub.y, and G.sub.z in the imaging region are therefore an essential condition for high image quality of nuclear spin tomographic apparatus.
Generally, the three gradients can be generated by magnetic quadrupoles. The fact that the coils for generating the gradients must be arranged inside the base field magnet must be taken into consideration in the design of nuclear spin resonance apparatus. Thus, sufficient space must be left for placing the human body to be examined.
An analytic derivation of the geometry of such coil systems can be obtained from the U.S. Pat. No. 3,569,823. Thus, the coils in the coil system are to produce a magnetic field which is developed into spherical functions which are as pure as possible. It is assumed here that the field-generating conductors are arranged on the outside and/or inside cylindrical surfaces of a hollow cylindrical support body. In such an arrangement, disturbances of the main spherical functions which are generated by the finite length of the conductors and their locations are analytically minimized.
The hollow cylindrical support body with the corresponding gradient coils can be inserted into a field magnet having an axis which coincides with the axis of the base magnet and which points, for example, in the z-direction of an orthogonal x, y, z coordinate system. The z-gradient G.sub.z is generated by two ring coils through which current flows in opposite directions. In order to generate the x-gradient G.sub.x two saddle-shaped coil pairs are placed on the support body. For the y-gradient G.sub.y, a corresponding system of four saddle-shaped coils is provided which are arranged opposite to the x-gradient coils either on the outer or inner cylindrical surfaces of the cylindrical support body, shifted by 90.degree. in the circumferential direction. The two pairs of individual coils of each coil set are arranged symmetrically with respect to an x-y plane which is oriented perpendicularly to the cylinder axis and extends through the center of the imaging region.
This linearity requirement can be met to a high degree by the coil arrangement described in the aforementioned DE-OS No. 28 40 178 or the published European Patent Application EP No. 21 531 A1. To this end, the gradient coil system of the known magnet coil arrangement is not arranged on a hollow cylindrical support body, but rather on a support body having a spherical shape. The examination axis corresponds to the axis of rotation of the support body, and extends in the z-direction of an orthogonal x, y, z coordinate system where the coordinate origin is placed in the center of the examination or imaging region. Instead of utilizing two toroidal individual coils for generating the field gradient G.sub.z in accordance with U.S. Pat. No. 3,569,823, a pair of toroidal individual coils is arranged at least substantially symmetrical with respect to the x-y plane through the center of the imaging region. The direction of current flow in the individual coils of one pair is opposed to the current flow direction in the individual coils of the other pair. In this arrangement, each of the z-gradient coils must have the same number of turns.
It is a problem with this arrangement, however, that the manufacture of such individual coils on the surface of a support body having a spherical shape is relatively expensive. The coils must be arranged on coordinates which must be positioned extremely accurately with respect to the coordinate origin so that the required linearity conditions can be met.
It is, therefore, an object of this invention to provide a gradient coil system which can be simply and inexpensively manufactured.
It is a further object of this invention to provide a gradient coil system which contains all of the advantages of the hollow cylindrical support body which is known in the art.